As a commercial storage battery, lithium-ion battery has the advantages of high energy density, long cycle life, no memory effect, and environmental friendliness, and has been widely used in portable digital devices, new energy vehicles, and energy storage devices. However, as the increase of the energy density requirement of lithium-ion batteries in the terminal market, the battery safety issue has become increasingly prominent, and has become a major bottleneck restricting the further development and innovation of lithium-ion battery technology. During use of the lithium-ion batteries, if the battery is abused (such as overcharged, overheated, or etc.) or subjected to violent impact, bending, piercing or other unexpected accidents, it is prone to cause fire or even explosion due to thermal runaway inside the battery. For the battery itself, the main cause of the thermal runaway is the poor thermal stability of the positive/negative active material, the separator diaphragm and the electrolyte. Meanwhile, some deficiencies in overall structural design of the battery are also the reason for the thermal runaway of the battery.
For this reason, in addition to actively developing new types of electrodes, separators, and electrolyte materials with excellent thermal stability, R&Ds have also devoted much energy to optimize the existing battery material system in order to improve the safety of the lithium-ion batteries without damaging or even improving the battery performance of the original system. Coating or depositing a thin layer of inorganic solid electrolyte composite coating with excellent lithium-ion conductivity and thermal stability on the positive/negative electrode plate or separator surface of the lithium-ion battery is an effective solution which could improve the cycle life and the safety features of the battery.
The coating or deposition of the inorganic solid electrolyte composite coating on the positive/negative electrode plate or separator surface of the lithium-ion battery is mainly achieved by two ways. The first one is physical vapor deposition methods such as magnetron sputtering, electron beam evaporation, pulsed laser deposition, ion beam sputtering or atomic layer deposition, and the second one is wet coating processes such as extrusion coating, gravure coating, screen printing, spray coating, or inkjet printing. Compared to various physical vapor deposition methods, preparing the inorganic solid electrolyte composite coatings by wet coating process undoubtedly has higher efficiency and lower cost, and is easy to seamlessly dock with the existing electrode plate preparation and separator coating process of the lithium-ion batteries, and therefore has more industrial application prospects.
For the wet coating process, the preparation of the inorganic solid electrolyte composite slurry is extremely important. The preparation method of the conventional inorganic solid electrolyte composite slurry is similar to the preparation method of the coating slurry of the lithium-ion battery electrode plate. The method includes firstly dissolving the required binder into the solvent, adding the inorganic solid electrolyte powder, the conductive agents and other components after the binder completely dissolved, and fully and evenly mixing the components together.
Considering that the requirement on energy density of lithium-ion batteries is increasingly increased in the terminal market, the thickness of the inorganic solid electrolyte composite coating applied to the positive/negative electrode plates or the separator surface of the lithium-ion battery should not be too thick (generally not more than 5 μm). However, after being prepared by the high temperature solid phase reactions, the particle size of many solid inorganic electrolytes such as garnet-type Li7La3Zr2O12 (LLZO), NASICON-type Li1+bAlbGe2−b(PO4)3 (LAGP) or perovskite-type Li3aLa2/3-aTiO3 (LLTO) is usually large (D50 is generally greater than 5 μm). Therefore, in order to prepare the inorganic solid electrolyte composite slurry by conventional method of firstly dissolving the binder into the solvent and then adding the inorganic solid electrolyte powder and other necessary components, it is necessary to firstly mill the inorganic solid electrolyte powder prepared by the solid-phase reactions with a relatively large particle size to an ultrafine powder having a sufficiently small particle size (D90 not more than 500 nm) by wet grinding (such as wet ball milling and/or sand milling). However, after the wet grinding process is completed, the drying of the inorganic solid electrolyte ultrafine powder, that is, the removal of the grinding solvent, is difficult to achieve. If the drying of the inorganic solid electrolyte ultrafine powder is accomplished by centrifugal separation, suction filtration, distillation or spray drying method, although the process is relatively simple, it is easy to cause the re-growth of the ultrafine powder. If the technique such as freeze-drying is used to effectively suppress the re-growth of the ultrafine powder, the cost and the energy consumption are both high, and the time spent is also long and therefore is not suitable for industrial production. On the other hand, if the composite slurry containing the dissolved binder and the large particle size of the inorganic solid electrolyte powder is directly subjected to the wet grinding process, the grinding efficiency will be low due to the presence of the binder, especially for the sand milling process, it is very easy to clog the grinding tank gap of the sand mill machine.